1. Field of the Invention
The present invention relates to a system for delivering gas from a liquefied state, and more specifically to delivering the gas at a controlled rate.
2. Description of the Related Art
Several manufacturing industries require the use of high purity, corrosive, liquefied gases to perform various manufacturing steps. For example, a reliable supply of ultra high purity electronic specialty gases is critical to maintaining productivity and manufacturing yield in the semiconductor industry. Exemplary semiconductor fabrication processes that require high purity gases include diffusion, chemical vapor deposition (CVD), etching, sputtering, ion implantation, etc.
Because some gases may be highly toxic and corrosive the gases may be contained carbon steel containers in a liquefied form. The carbon steel containers are typically gas cylinders that are configured to contain and deliver the high pressure gases in a safe manner.
Manufacturing processes may require that the gases be provided in a controlled manner at a determined flow rate. One problem with providing gases from cylinders is that the temperature of the liquefied gas may decrease with time. The temperature drop in the liquid may decrease the vapor pressure of the gas. The dropping vapor pressure may adversely affect the rate of gas flow from the cylinder.
One solution to maintaining gas flow rates is to heat the cylinders, thereby maintaining the temperature and vapor pressure in the cylinder at a desired level. Heating the cylinders usually involves placing the cylinder in a tub-shaped unit containing heating elements. The heating elements may input heat into the cylinder through the cylinder walls. However, such solutions require hoisting the cylinder from its transportation pallet to place the cylinder in a heating tub. The large size and weight of the cylinder may make such hoisting very tedious. Furthermore, because the cylinders typically contain toxic gases such as ammonia, the hoisting of the cylinder may increase the risk of dropping and damaging the cylinder, which may result in the leakage of the toxic gases.
Other prior art solutions include induction heating of the cylinder by wrapping a heating coil or heating jacket around the cylinder. However, such heating jackets have proven to be thermally inefficient.
Accordingly, what is needed are improved methods, systems, and apparatus for safely and efficiently providing liquefied gases at a controlled rate.